Effect of carbonate and biological macromolecules on formation and properties of hydroxyapatite

Abstract

Amorphous calcium phosphate (ACP) was transformed at 25° to hydroxyapatite (HA) in horse and bovine serum; solutions of serum-protein fractions in tris-HCl buffer (pH 7.4), and pH 7.4 buffers containing from 0.1 to 10 times physiological CO₃ ²⁻ concentration. The ACP-to-HA transformation was slower in whole serum and serum fractions than in control buffer solution. The observed adsorption of serum proteins on ACP and HA probably inhibits both the dissolution of the ACP particles and the growth of HA crystals. After 72 h all transformations were complete as determined by X-ray diffraction. The HA crystal dimensions decreased with increasing CO₃ ²⁻ but the shape, as shown by X-ray linewidths, was relatively constant up to about 4% CO₃ ²⁻. At 15% CO₃ ²⁻ the crystals were more equiaxial and less needle-like in habit. The radial distribution function (RDF) of HA with 3.7% CO₃ ²⁻ is less well resolved than the RDF of HA with ambient CO₃ ²⁻ (1.1%). The peaks are less sharp and their amplitude falls more rapidly with increasing atomic separation than for low CO₃ ²⁻-HA. These effects show that CO₃ ²⁻ decreases the regularity of the atomic arrangement when incorporated in HA. The rapid decrease, with increasing CO₃ ²⁻ content, of the IR splitting of the P−O bending mode of CO₃ ²⁻-HA is attributed to reduced crystal size and possibly to a perturbation of the crystal field due to CO₃ ²⁻-induced lattice distortion. Finally, for bone mineral, it is probable that the poor resolution of the X-ray and IR patterns is due, in large part, to small crystal size and internal disorder caused by CO₃ ²⁻.